1. Field of the Invention
The present invention relates to an acetabular cup assembly for a hip prosthesis. More particularly, this invention relates to a bone screw for fixing the acetabular cup to the acetabulum.
2. Description of the Prior Art
Known total hip prostheses include prosthetic femoral components and acetabular cup assemblies in which a spherical head integral with the femoral component can rotate freely in a spherical cavity in the acetabular cup. This allows a prosthetic assembly to replace the normal articulation of the hip.
The prosthetic acetabular cups of these known prostheses are generally fastened in the patient's iliac bone either directly by being screwed into the acetabulum, or through the use of bone screws, or indirectly, by means of an acrylic bone cement.
Examples of acetabular cups which are threaded are shown in U.S Pat. Nos. 4,062,891, 4,795,469, 4,813,961 and 4,822,367. In these acetabular cups an external screw thread is made of metal and its implantation requires prior preparatory work on a large area of a healthy part of the iliac bone by tapping the bone. This step risks weakening this bone and thus compromises the seat of the prosthetic acetabulum.
With bone cement, an acetabulum made of a polyolefin, for example polyethylene, and comprising a number of cavities or external protuberances intended to ensure its fixation, must be introduced immediately after the acrylic bone cement has been applied. The cup must be firmly maintained under pressure while the cement polymerizes in situ. This involves delicate handling of the cup and extends the duration of the operation. This process also produces the setting of a intermediate layer of cement which is never fully compatible with the bony tissue.
U.S. Pat. Nos. 4,792,337 and 4,871,368 are examples of the use of bone screws in the fixation of an acetabular cup prosthesis to a prepared acetabulum. The use of bone screws has been found to overcome many of the problems of the prior art fixation systems. However, it has been found that an acetabular cup fixed only with bone screws can subside into the cavity or part of the cavity formed in the acetabulum thus causing the threaded coupling to loosen at least in certain areas.